The Kinematics of the Permitted C II $λ$ 6578 Line in a Large Sample of Planetary Nebulae

Abstract: We present spectroscopic observations of the C II $\lambda$6578 permitted line for 83 lines of sight in 76 planetary nebulae at high spectral resolution, most of them obtained with the Manchester Echelle Spectrograph on the 2.1\,m telescope at the Observatorio Astron\'omico Nacional on the Sierra San Pedro M\'artir. We study the kinematics of the C II $\lambda$6578 permitted line with respect to other permitted and collisionally-excited lines. Statistically, we find that the kinematics of the C II $\lambda$6578 line are not those expected if this line arises from the recombination of C$^{2+}$ ions or the fluorescence of C$^+$ ions in ionization equilibrium in a chemically-homogeneous nebular plasma, but instead its kinematics are those appropriate for a volume more internal than expected. The planetary nebulae in this sample have well-defined morphology and are restricted to a limited range in H$\alpha$ line widths (no large values) compared to their counterparts in the Milky Way bulge, both of which could be interpreted as the result of young nebular shells, an inference that is also supported by nebular modeling. Concerning the long-standing discrepancy between chemical abundances inferred from permitted and collisionally-excited emission lines in photoionized nebulae, our results imply that multiple plasma components occur commonly in planetary nebulae.